Polyimides have proven valuable in the formation of flexible printed circuits. Flexible printed circuits can significantly reduce the weight of the electronics making them important in the portable electronics industry. Flexible printed circuits also reduce the costs associated with assembling the electronics package by reducing the assembly operation and allows for testing of the circuits prior to further assembly.
Typically, a flexible printed circuit is a laminated material that consists of a dielectric layer and a conductor layer. A copper foil is often used as the conductor layer and a polyimide layer is often used for the dielectric layer.
The polyimide is typically processed from a poly amic acid precursor to form a thin film or a coating cast onto the copper film. The poly amic acid precursor is typically in an organic solvent such as N-methylpyrrolidinone (NMP). However, processing on a large scale production line requires that the solvent be removed at rate much faster than what can be achieved when using NMP as a solvent. Some efforts have involved adding xylenes to NMP to facilitate solvent removal. Solvent removal for the xylene/NMP system is not rapid enough to be utilized on a large scale production line.
Other efforts have employed a solvent system containing acetone. This method requires the use of a preheater in gradual heating regimes to remove a significant portion of the solvent prior to forming the polyimide layer.